Apple has patented a set of methods for lighting a display using solar power. It seems that Apple is looking at ways of using existing light sources to make its displays visible.
Currently the best method is using internal reflectance, which is how e-ink is visible in sunlight. Apple's method involves channelling and reflecting the light to the edges or back of the display. According to the Patent: "electronic devices may not be used effectively in certain physical settings, such as, for example, where the external or ambient light is brighter than the illumination of the display screen."